The present disclosure relates, generally, to circuit protection devices. More particularly, it relates to fusible substrates that fracture upon reaching a predetermined temperature to provide overcurrent protection.
Existing fuses have several issues regarding both failing when they should not fail and not failing when they should fail. Severe surges such as lightning strikes should cause the fuse to fail; however, the fuse needs to withstand smaller surges such as those that occur upon initial current flow through the circuit. Brief, severe surges are not the only condition that should cause fuse failure. A phenomenon known as a sneak current can also overload a circuit resulting in fuse failure. Sneak currents occur by an incident such as a power line falling on top of a telephone line, which induces a low level increase in current that exceeds the capacity of the circuit. Present fuse technology allows for complete fuse failure within 30 seconds under a sneak current. Although this time appears to be short, circuit damage can still occur within these 30 seconds.
A phenomenon known as arcing can also be problematic in that it allows the fuse to carry current after the onset of melting. The fuse element begins to melt at its hottest spot, typically in the middle of the fuse. Metal vapor remains in the air gap between the melted ends. The metal vapor continues to conduct the current across the gap which is fed by the voltage in the circuit. The arc generates a plasma of ionized gases which then takes over the current. The ionized arc creates more heat, pressure, and current in the gap.